1. Field of the Invention
This invention relates to a superconducting current limiting device that limits overcurrent flowing in an AC path, using a superconducting coil. In more detail, this invention relates to a superconducting current limiting device capable of limiting overcurrent without generating a quench phenomenon whereby the superconducting coil performs an abrupt phase transition from superconducting condition to ordinary conducting condition.
2. Description of the Related Art
If a three-phase short-circuit or "earth drop" occurs in an AC current path such as a distribution line, a fault current i.e., an overcurrent of several tens of kA may result, causing tremendous damage to the distribution system and equipment. Superconducting current limiting devices have been devised which make use of superconductivity as one current limiting technique for instantaneously detecting and controlling such fault currents.
FIG. 8 is a circuit diagram showing the layout of a prior art superconducting current limiting device as described above disclosed in early Japanese patent disclosure number H 2-168 525. In this FIG. 10 is a current source, 20 is an interrupter, 30 is a current limiter, 3a is a superconducting current limiting coil (hereinbelow called a current limiting coil), 3b and 3c are trigger coils, 3d is a switch, 3g is a quench sensor, 4 is a load, 21 is a current transformer for detecting the total circuit current, 22 is a current transformer for detecting the loop current of the trigger coils, 23 is a control power source, 24 is a loop current phase detection circuit, 25 is a switch for connecting or isolating two-terminal voltage phase detection circuit 26 of the trigger coils to or from the main circuit, and 27 is a phase detector for comparing and detecting the phase difference of current and voltage of the trigger coils.
A simple explanation of the operation of this current limiting device will now be given. In the steady-state condition, the circuit current flows through non-inductive superconducting (zero resistance) trigger coils 3b and 3c, so that power continues to be supplied normally to load 4. Then, if load 4 is short-circuited etc., an excessive fault current flows in the circuit. When the value of this fault current reaches the critical current value of the superconducting wire constituting the trigger coils, the trigger coils are quenched i.e. execute a transition to high resistance. As a result, the fault current is suppressed, and is commutated to a current limiting coil of lower impedance from the trigger coils. The trigger coils are thereby converted into ordinary conductors and so generate heat which dissipates the coolant and also raises the temperature of the superconducting wire. Switch 3d opens immediately after commutation of the fault current from the trigger coil current to the current limiting coil, so as to suppress evaporation of coolant. It also assists in restoring superconduction of the trigger coils so that they are ready to respond to the next fault occurrence, by speeding up the rate of cooling of the trigger coils. Restoration of superconduction of the trigger coils is detected by respective phase detectors 24 and 26 and their comparator 27. When both conditions, i.e. the system fault has been recovered and the trigger coils have returned to superconduction, have been satisfied, switch 3d closes and the equipment thereby returns to the stand-by condition.
The prior art superconducting current limiting device described above is subject to the following problems.
(1) Since the prior art superconducting current limiting device described above is of the type in which the superconductor is quenched, there are the problems that there is a considerable Joule loss Pj on current limiting operation (quenching), and the amount of consumption (vaporization) of coolant used to cool the superconductor is increased. PA0 (2) Once current-limiting operation has been performed, some considerable time is required to restore the equipment to its original superconducting condition. The equipment therefore cannot be used with high frequency. PA0 (3) A further problem is that, in a quenching-type superconducting current limiting device, the current limiting condition is determined solely by the quenching current of the trigger coil, so the condition for its current limiting action cannot be selected over a wide range. PA0 (4) Yet a further problem is that, since the current limiting operation current is determined by the critical current value of the trigger coil, it can only have the value that was designed beforehand. Coil standardization is therefore difficult.
If the quenching resistance Rq of the trigger coils is fixed, the value of this Joule loss Pj is proportional to the square of the source voltage V, as shown by the following equation. EQU Pj=(V.sup.2 /Rg)t (J) (1)
where t is the conduction time (seconds) after occurrence of quenching.
Consequently, if the circuit voltage is increased, this results in an increase in the loss generated on quenching and an increase in the consumption (evaporation) of coolant used to cool the superconductor. As an example, assuming that a short-circuit current generated in a circuit of 6.6 kV is current-limited in one cycle (20 msec) by a trigger coil of quench resistance 30 .OMEGA., the loss which then occurs can be found as follows from equation (1). ##EQU1##
If we assume that the coolant is liquid helium, about 12 liters of liquid is instantaneously evaporated. This evaporated helium expands to a volume of several hundred times, so the cryostat in which the superconducting current limiting device is received must be of large size and rigid structure so as to be capable of withstanding the internal pressure which is then generated. A further problem is that means must be provided so that the evaporated helium can be discharged to outside.
Usually, when such a superconducting coil is quenched, the superconductor rises in temperature by some tens to some hundreds of K. Consequently some tens of seconds to some minutes are subsequently required to cool the conductor of the superconducting coil down to the critical temperature for superconduction i.e. about 10K. During this period, the equipment cannot function as a current-limiting device. One method of dealing with this problem that has been considered is to reduce the recovery time by installing two trigger coils which are used alternately. However, this is inefficient as it increases the size of the equipment and complicates its control.
Specifically, when a current limiting device is applied to various systems, the system protection conditions must be chosen optimally taking into account the relationship between the level (i) of overcurrent and the time (t) for which it passes. However, in the conventional quenching type current limiting device, current-limiting or non-current limiting mode is determined solely by the value of the overcurrent level (i). Thus, the conventional equipment is lacking in flexibility in its application to different systems.
Specifically, in linking systems of different types, it is necessary to have the operation current value of the individual current limiting devices slightly different from each other (selectable limiting current/selectable cut-off). However, with the prior art superconducting current limiting devices, different operating current can only be achieved by altering the critical current value of the superconducting wire constituting the coil. A large number of different coil types are therefore required.